I'm a huge fan of glitterdust and suggestion. Both of them are flexible spells.
Glitterdust has its two effects, which is very nice: a great effect vs. invisible creatures, and a decent effect against noninvisibles. Your melee buddies will love you for blinding the enemies, and will adore you for revealing improved-invisibility enemies.
Suggestion has only the one effect, but boy howdy is it a great one. "We're really tough; you'd be surrender now!" "Hey, wizard, our barbarian can't wrestle worth a damn, I sure hope you don't decide to tackle him, wink wink!" "Oh, look at the cowardly blackguard, hiding behind his big armor and shield--why don't you take off that armor and fight like a man?" And that's only the combat potential.
Sure, outside of combat, fascinate can cover the need for suggestion; but during combat, suggestion rocks.
Daniel